This invention relates to systems and processes for molding relatively thick walled articles form fiber reinforced thermoplastics which perform at extremely high temperatures and stresses. The processes and apparatus disclosed herein may also be utilized for molding of thermoset resins.
In my issued US patents, including U.S. Pat. No. 6,984,352 and U.S. Pat. No. 7,223,087, I disclose methods for creating compression molds for use in the compression molding of polymers using microwave energy to heat the polymer material to its melting point. The molds and processes disclosed therein are particularly well adapted for molding plastic polymers and composites having a relatively high operating temperature, including such high performance polymers as those sold under the trademarks PEEK®, TORLON®, SEMITRON®, DURATRON®, CELAZOLE®.
In my U.S. Pat. No. 7,122,146, I disclosed methods and apparatus for injection molding of polymers utilizing microwave energy. This process is intended for molding thick walled parts from polymer in the form of pellets or powders, which provides a higher quality molded product compared to parts molded by compression molding. The mechanical properties of injection molded parts are usually higher than those of compression molded parts. However, the capital costs for producing such injection molding systems using microwave energy to plasticize the material to be injected can be relatively high.
The idea of a variable volume mold cavity is known in the prior art. For example, in an injection-compression molding (ICM) process, two mold halves are maintained in a slightly open alignment as molten plastic is injected into the mold. Once the required amount of plastic to form the molded part is injected into the mold, the mold halves are advanced toward each other to close the mold and to provide improved flow of the melt into the all portions of the mold cavity to get a dense molded part without air voids. In contrast to injection-compression molding, Nomura et al. in U.S. Pat. Nos. 6,010,656 and 6,457,917 discloses a process for injecting molten resin into a variable cavity mold under pressure while the mold cavity is maintained at a first volume and then at the end of the injection cycle, expanding the volume of the mold cavity to rapidly decrease the pressure acting on the molten plastic, causing the molten plastic or resin to expand due to its internal gas pressure to obtain a relatively light product, low density product. A mat of glass fibers is preferably positioned in the mold to obtain a very light fiber-reinforced product of low density.
In both cases, the variation of the mold cavity begins either after completion of the injection or when it almost completed. In either cases, there exists a period of time when the melt is not fully compressed and it may expand, forming pores or voids in its volume. The formation of such air voids or porosity may be caused either by air trapped in the melt or due to hot gases of the melt. In ICM such air voids or porosity is removed from the melt by significant mold closing pressure and due to relatively small thickness of molded product and improved thickness to flow length relation. Neither of the described techniques are suitable for use in the injection molding of parts having relatively large cross-sections or thick walls. In thick walled parts, any air voids or pores formed in the injected plastic are likely to be trapped therein. As a result the molded part will be rejected.
There remains a need for systems for providing for the relatively rapid and uniform heating of high performance engineered plastics having relatively high operating temperatures using conventional heating sources such as electric heaters. There further remains a need for such systems for supplying molten plastic for injection molding applications in which the molded parts are of high quality and relatively free from air voids and pores. There further remains a need for such a system in which the plastic to be injected can be supplied from a plasticizing vessel designed to minimize oxidation and degradation of the plastic material therein particularly during the process of filling and emptying the plasticizing vessel. There further remains the need for operation of such system in continuous mode without interruption of the heating process while adding fresh plastic to the plasticizing vessel.